1. Field of the Invention
The present invention relates to an amorphous silica filler, and more specifically to an amorphous silica filler which exhibits excellent handling property and processability and which, when added to a resin film, exhibits excellent dispersing property, transparency and anti-blocking property, and which makes it possible to obtain a resin film having excellent scratch-resistant property that does not get scratched on the surfaces even when the films are rubbed by each other.
The present invention further relates to a filler for heat-sensitive recording papers, and more specifically to an amorphous silica filler for heat-sensitive recording papers that can be excellently applied onto the heat-sensitive recording papers, that makes it possible to form a vivid heat-sensitive image without fogging on the ground, and that does not permit scum to adhere on the thermal head.
2. Description of the Prior Art
The amorphous silica fillers include the so-called dry-method silica and the wet-method silica, and are used for paints, information-recording papers, rubbers, resin molded products and like applications depending upon their properties.
The amorphous silica exhibits properties that vary greatly depending upon the manufacturing method. In particular, the latter wet-method amorphous silica exhibits widely varying properties if conditions such as concentration, temperature, pressure, time and reaction method are changed in neutralizing an alkali silicate with an acid.
For instance, there have been known a silica hydrogel (Japanese Patent Publication No. 13834/1973, Japanese Patent Laid-Open No. 16049/1988) obtained by gelatinizing (spraying into a gaseous medium) within a short period of time a silica sol formed by neutralizing an alkali silicate with an acid, and a hydrated silica gel (Japanese Patent Publication No. 1764/1990) having a water content of 20 to 50% by weight obtained by washing with an acid a silica hydrogel hardened with an acid to obtain a silica hydrogel having pH of 2.5 to 5, followed by drying and pulverizing.
In addition to the above amorphous silica, furthermore, there have heretofore been widely used inorganic fine particles as an anti-blocking agent for resin films. For instance, there have been proposed a method (Japanese Patent Publication No. 41099/1974) of improving transparency and anti-blocking property by adding fine powdery silica whose silanol groups on the surfaces thereof have been substituted by oleophilic groups to a resin, a method (Japanese Patent Publication No. 16134/1977) of improving anti-blocking property by adding zeolite particles to biaxially undrawn polypropylene film, and a drawn polypropylene film (Japanese Patent Publication No. 58170/1988) having improved transparency, slipping property and anti-blocking property obtained by adding fine powdery silica having an apparent specific gravity of 0.1 to 0.2 g/cm.sup.3 and a specific surface area of smaller than 150 m/g.
Furthermore, a heat-sensitive recording paper has heretofore been used in facsimiles, printers, data communications, computer terminals, measuring instruments and copying machines which use a thermal head, a heat pen, an infrared-ray lamp or a laser as a source of heat, the heat-sensitive recording paper being provided with a recording layer that comprises a color agent such as a leuco-pigment and a color coupler such as phenols that develop a color upon hot-contact with the color agent, which are contained in a binder.
In making a record on the heat-sensitive recording paper by the heat by bringing a recording head into contact with the recording layer, the components in the recording layer melt and stick resulting in the adhesion of scum onto the recording head and the development of sticking. In order to prevent these problems, various fillers are contained in the recording layer. When the amorphous silica is contained in the recording layer as a filler for the heat-sensitive recording paper, however, the reaction is promoted between the leuco-pigment and the phenols due to the surface activity of silica giving rise to the development of ground color (ground fogging).
In order to prevent the above problem, the present inventors have proposed in Japanese Patent Publication No. 1030/1990 a filler for heat-sensitive recording papers comprising fine particulate amorphous silica having a particle size distribution of secondary particle sizes in which particles of a size of smaller than 4 .mu.m as measured by the centrifugal sedimentation method are contained in amounts of more than 90% by weight of the total weight, having a BET specific surface area of 10 to 100 m.sup.2 /g, and having a bulk density of 0.14 to 0.30 g/ml.